Ans. When pulverized coal (bituminous/lignite is burnt in the boiler of a thermal power station, a part of ash falls down at the bottom of the boiler and is known as bottom ash. Whereas, the major portion of the ash comes out alongwith the flue gases and is collected through electro static precipitator or filter bags or other means before allowing the exhaust gases through escape the chimney, this part of ash is generally known as ESP ash. The un-utilised ESP ash and bottom ash are taken to lagoons known as ash ponds for deposition. The ash deposited in the ash pond is known as pond ash. In some cases this ash is deposited in the form of a mount or hillock and is known mound ash.

Fly ash or pulverised coal ash or pulverised fuel ash in general term represents all types of ashes produced in the thermal power station unless otherwise specifically referred to as ESP ash(chimney ash, dry fly ash), bottom ash, pond ash and mound ash.

Small part of ash is also collected in the economizer & pre-heater. This ash should generally be discharged alongwith bottom ash, being a coarse ash. However, at number of power stations it is discharged with ESP ash.

Q.4. Can all parts of fly ash are useful for every application?

Ans. Broadly speaking all parts of fly ash can be used for every application . However, the effectiveness and efficiency of uses varies. Each Part of fly ash is best suited for specific applications, as detailed below:

1. ESP ash/Chimney ash/dry fly ash:

It is best suited for manufacture/ part-substitution of cement and the applications, wherein fly ash is used as cementatious/pozzolonic applications. Such as manufacture of fly ash bricks (without clay), blocks, pavers, pre-fabricated items, roofing sheets, tiles, tree guards etc.

2. Bottom ash:

Being a coarse ash, it is an excellent substitute of soil for geotechnical applications such as filling/reclamation of low lying areas, construction of dykes, road and fly over embankments etc.

After processing for removal of carbon and grinding to sand size, it is a good part-substitute of sand in mortar and concrete.

3. Pond ash/mound ash:

Its a medium grain size ash, mixture of ESP ash and bottom ash. It is best suited for use in agriculture, waste land development and forestry applications. It’s also a good material for geotechnical applications as a substitute of soil.

Other important utilisation of pond ash is in manufacture of clay bricks. When pond ash is mixed with good clay to the extent of 30-80% depending on the clayness of clay, it improves the quality of clay brick (now clay fly ash brick), reduces breakage at the kiln as well as during transit/use and also reduces fuel consumption in the kiln.

For ceramic , metallurgical and other high value added applications of fly ash one of the three portions of fly ash is stated above would be best suited depending on the properties of fly ash that contribute to that particular application. Other portions of fly ash can also be utilized but the result/impact would be sub-optimised.

Ans. ESP ash/chimney ash/dry fly ash is recommended for this use. However, pond ash or bottom ash can also be used but the results (strength of the bricks) would not be as good as in case of ESP ash.

Q.6. Can ESP ash/chimney ash/dry fly ash be used for agricultural application?

Ans. Such a fine ash in dry condition is not advisable to be used in agriculture application as its transportation, application (spreading and ploughing) would be costlier, difficult and may cause air pollution.

Pond ash is best suited for this application as it contains some moisture and does not become air borne easily.

Q.7. Is the quality of pond ash same at all locations in the ash pond?

Ans. No. The quality of ash in different zones of pond ash is different.

The ash deposited within about 100 meters of ash slurry discharge point in the pond is coarser ash as compared to the ash deposited within about 100 mtrs. of water overflow discharge point in the ash pond. The ash deposited in between these two areas is of medium particle size.

Further, during the process of settling of ash in the pond, natural segregation process takes places. Coarser ash settles down first and the final particle in the top layer. With the general practice intermittent filling of ash ponds, repetitive layers of about 1 meter thickness get developed with segregated particles.

Q.8. Can same pond ash be used for all types of soils for agriculture applications?

Ans. For optimum results the fly ash needs to be selected based on physical and chemical properties of soil and the cropping pattern. However, all bituminous coal fly ashes(Class F fly ash) give good results in all soils and all cropping patterns.

Application of acidic fly ashes (which is rarely available in India) should be avoided. For best results the selection of fly ash and dosage be done under the advice of agriculture fly ash experts. The selected fly ash and the dozes should maximise the supply of micro nutrient required for the cropping pattern deficient in the soil and should improve the soil texture/physical properties of the soil.

Q.9. Will cementatious property of fly ash will create hard lumps of soil when used for agriculture applications?

Ans. No it will not.

Fly ash is a pozzolonic material which behaves like a cement in presence of lime/cement and water. In common soils fly ash will not behave like cementatious material. It should not be used in the soils where lime treatment is being given.

Q.10. Is fly ash radio active? Is it harmful ?

Ans. Each material, including fly ash is radio active. It is the degree of radio activity that determines its impact. The radio activity level of fly ash is 1/10th to 1/20th that of the level that cause any harm. This has been tested and certified by Department of Atomic Energy, Government of India.

Radio activity levels of fly ash bricks, agriculture soils treated with fly ash as well as the agriculture produce grown on fly ash treated soils have been tested for radio activity and are certified as very safe.

Q.11. Does fly ash contain heavy and toxic metals? What is the impact of these elements on the environment or to the public.?

Ans. Fly ash contains traces of heavy and toxic metals which are generally far below the level that can cause any impact on the environment or public during its normal use.

Fly ashes having high contents of toxic/heavy metals (such fly ashes have yet not been found in India ) may be used/deposited under expert advice.

Q.12. Is mixing of fly ash in cement or concrete harmful?

Ans. Mixing of fly ash of recommended quality and quantity in cement or concrete improves the quality of cement as well concrete. The well laid structures with fly ash concrete/cement are stronger and durable. Such structures are also less prone to damage by coastal and chemical environments.

Q.13. Can bottom ash be used as a part substitution of sand?

Ans. Yes, pond ash after removal of un-burnt carbon and grinding to sand size can be used as part substitution of sand.

Q.14. Can fly ash be used for back filling of open cast mines?

Ans. Yes fly is a good material for back filling of open cast mines. It can be used as a structural fill material that is by laying in layers and with appropriate compaction. It can also be used alongwith overburden with intermittent mixing i.e. if 10 dumpers are bringing over burden, two dumpers can be employed bring in fly ash and all these materials can be backfilled in a regular manner as per the existing practice of back filling. The fly ash would get automatically intermixed and fill up the voids in the over burden material, providing it stability and nutrition as well as water holding capacity for aforestation.

Q.15. Can fly ash be used, in place of sand, for stowing of underground mines?

Ans. Yes, fly ash is a good material to substitute sand for back filling of open cast mines. Coarser portions of fly ash that is bottom ash and coarse ash from pond ash are even better materials than sand for this purpose. Fine ash particles that take relatively longer time for settlement, can also be settled faster with application of appropriate technology.

Q.16. What is fly ash beneficiation?

Ans. Processing of fly ash to make it more suitable for a specific utilization is known as beneficiation of fly ash.

Q.17. Can fly ash be ground to improve its fineness?

Ans. Yes, the fineness of the fly ash can be improved by grinding.

Q.18. Does grinding of coarse fly ash improve its reactivity?

Ans. Yes, the grinding of fly ash increases its surface area and thus generally improves its reactivity. However, reactivity of fly ash is a function of many parameters and not only that of surface area. The cumulative effect of reactivity is a detailed subject. Application of appropriate technology for fly ash beneficiation would give best results in terms of reactivity enhancement.

The Information provided through this website is from varying sources. It's accuracy and usefulness to your requirements may be assessed . C-FARM is not responsible for any direct or consequential effects. The information can be disseminated in public interest with acknowledgement of the source: (www.c-farm.org)